In recent years, with the mobilization and high functionality of electronic equipments, the secondary battery, which is a power source, has become one of the most important parts. In particular, Li-ion secondary battery has become the mainstream in place of conventional NiCd battery and Ni—H battery, due to its high energy density obtained from the high voltage of the cathode active material and anode active material. However, Li-ion secondary battery by the combination of lithium cobalt oxide (LiCoO2)-type cathode active material and carbon-type anode active material mainly composed of graphite, which is normally used as Li-ion battery presently, is incapable of sufficiently supplying the amount of electricity required for today's high-functionality and high-load electronic parts, and is not able to fulfill the required performance as a portable power source.
The theoretical electrochemical specific capacity of the cathode active material is generally small, and manganic acid-type lithium and nickel oxide-type lithium currently used beside cobalt oxide-type lithium, as well as iron phosphate-type lithium that is being studied to be put in practical use, all remain to show smaller values than the theoretical specific capacity of current carbon-type anode active materials. However, the carbon-type anode active material, of which its performance has been rising little by little every year, is also approaching the theoretical specific capacity, and it is becoming impossible to anticipate large improvement in power source capacity with the current combination of cathode and anode active material systems. There appears to be a limitation in meeting requirements for high-functionality and long term mobile running of electronic devices, for loading on to industrial applications such as electric power tools, uninterruptible power sources, and electric storage devices, for which adoption is spreading, and for electric-powered vehicles.
Under such circumstances, metal-type anode active materials are being examined for application, as a method to dramatically increase the electric capacity than that currently possible, in place of the carbon (C)-type anode active material. Such method enables several to ten times the theoretical specific capacity of the current carbon-type anode. These active materials are germanium (Ge), tin (Sn) or silicon (Si)-type materials. In particular, Si has a specific capacity that is comparable to that of metallic Li, which is said to be difficult to put to practical use, and is thus, being the center of study.
However, in the present situation, because the specific capacity of the cathode active material side is low, the large theoretical specific capacity of Si is actually not being put to use in batteries. The per-unit-mass theoretical specific capacity of the layered or tunnel-like compound complex oxides, which can serve as an intercalation host of Li that is being considered for utilization as a cathode active material, is slightly over 150 mAh/g at most, which is less than half the specific capacity of the present carbon-type anode active material, and is actually 1/20 or less against the theoretical specific capacity of Si. For this reason, the examination of substance systems in aim to achieve higher capacity of cathode active material is also needed. As a candidate for new cathode active material, studies on lithium transition metal silicate-type compound, which is expected to exceed 300 mAh/g, or twice the conventional value, depending on its components, are beginning (for example, Patent Document 1 and Non-patent Document 1).